1. Field of the Invention
This invention relates to a method for measuring the thickness of an ice sheet. More particularly, this invention relates to a method for continuously measuring the thickness of an ice sheet in an arctic environment.
2. Description of the Prior Art
As the exploration for oil and gas expands into the offshore arctic regions, it is occasionally necessary to measure the thickness of a floating ice sheet for various reasons such as the development of design criteria for offshore structures.
In the past, ice thickness has been determined directly by borings or indirectly by radar, magnetic induction or ice temperature measurements. However, while each of these methods has its particular advantages, each method does have certain distinct disadvantages.
Drilling a hole requires that the hole be kept clean and open. In addition, measurements must be made at predetermined intervals to continuously monitor the thickness of the ice sheet.
The radar method uses a source and receiver located above the ice sheet. A signal is refracted by the interface between the bottom of the ice sheet and the sea. However, occasionally a second interface between the ice sheet and water on top of the sheet resulting from melting ice interferes with the results.
The magnetic induction method is described in a paper by P. Hockstra and A. Santorelli entitled "Low Frequency Methods for Measuring Sea Ice Thickness" presented at the International Workshop on Remote Estimation of Sea Ice Thickness in Newfoundland, Canada on Sept. 24-25, 1979. Basically, this method uses a magnetic dipole transmitter and receiver spaced apart, yet located on the top of the ice sheet. The transmitter is used to induce an eddy current in the ice sheet which the magnetic dipole receiver measures. The thickness is determined by comparing the measurement made by the receiver with a reference point such as the measurement of an eddy current in open air. However, indications are that the measurements made by the receiver may vary as much as 35% from the actual thickness of the ice sheet.
The ice temperature measurement method uses a number of temperature measurements through the depth of the ice sheet and sea water below. In this method, the location of the bottom of the ice sheet is determined from the ice temperature gradient and the water temperature. However, the ice temperature method is unreliable because it depends on the occurrence of both a significant ice temperature gradient and a constant water temperature, either of which conditions may fail to exist. The ice temperature method also places a heavy burden on the field data acquisition program because of the large number of temperature measurements which must be made.
Therefore, the need exists for an improved method for measuring the thickness of an ice sheet which is inexpensive, accurate and reliable.